1. Field of the Invention
The present invention relates to methods of fabricating extended arrays of silicon wafer subunits, and more particularly to methods of forming pagewidth thermal ink jet printhead arrays from discrete thermal ink jet printhead subunits.
2. Description of Related Art
With the increased interest in rastor scanners, both to read and write images, has come renewed demand in the art for an economical full width scanning array. In the current stage of scanner technology, the art is without a commercially acceptable and economically feasible method of producing very long unitary scanning arrays, that is, single arrays of sufficient linear extent and with the requisite number of image processing elements to scan an entire line at once with a high image resolution. In this context, when speaking of scanning arrays, there are both image reading arrays which comprise a succession of image sensing elements such as, for example, photosites and supporting circuitry, to convert the image line to electrical signals or pixels, and image writing arrays which comprise a succession of light producing or other elements employed to produce images in response to an image signal or pixel input.
The prior art has faced this failure or inability to provide long full width scanning arrays with various proposals. These include optical and electrical arrangements for overlapping plural shorter arrays and abutting short arrays together in end-to-end arrangements. However, none of these proposals has met with any great degree of success. For example, in the case of abutting smaller arrays together, due to the difficulty of exactly aligning and mating the array ends with one another, losses and distortion of the images often occur.
A similar problem arises with thermal ink jet printing systems. Thermal ink jet printing systems use thermal energy selectively produced by resistors located in capillary filled ink channels near channel terminating nozzles or orifices to vaporize momentarily the ink and form temporary bubbles on demand. Each temporary bubble expels an ink droplet and propels it towards a recording medium. The printing system may be incorporated in either a carriage type printer or a pagewidth type printer. The carriage type printer generally has a relatively small printhead, containing the ink channels and nozzles. The printhead is usually sealingly attached to a disposable ink supply cartridge and the combined printhead and cartridge assembly is reciprocated to print one swath of information at a time on a stationarily held recording medium, such as paper. After the swath is printed, the paper is stepped a distance equal to the height of the printed swath, so that the next printed swath will be contiguous therewith. The procedure is repeated until the entire page is printed. For an example of a cartridge type printer, refer to U.S. Pat. No. 4,571,599 to Rezanka. In contrast, the pagewidth printer has a stationary printhead having a length equal to or greater than the width of the paper. The paper is continually moved past the pagewidth printhead in a direction normal to the printhead length and at a constant speed during the printing process. Refer to U.S. Pat. No. 4,463,359 to Ayata et al for an example of pagewidth printing and especially FIGS. 17 and 20 therein.
Thermal ink jet printers include printheads, such as side shooter printheads shown in FIG. 1 and described in U.S. Pat. No. 4,601,777 to Hawkins et al (the disclosure of which is herein incorporated by reference). It is desirable to form these printheads having the width of a page to enable high speed printing to be performed. One method of forming these pagewidth printheads, illustrated in FIG. 1, involves butting together a plurality of printhead subunits S.sub.1, S.sub.2, S.sub.3 to form a printhead array having the length of a pagewidth. With the sideshooter printhead, each printhead subunit includes a heater plate 2 having a series of resistive heater elements 3 located on an upper surface thereof attached to a channel plate 4 having a series of channels 6 located on a lower surface thereof and corresponding in number to the resistive heater elements. The end of each channel 6 forms a nozzle from which a drop of ink is outputted upon actuation of the corresponding resistive element 3. One method of forming such pagewidth printheads from an array of printhead subunits involves flipping each printhead subunit S.sub.1, S.sub.2, S.sub.3 upside down and physically butting it to an adjacent printhead subunit. The channel plate 4 is etch delineated to be less wide than the heater plate 2, which can be precisely delineated by a precision dicing saw. With such a configuration, the channel plate plays no part in the physical butting process and only the precisely diced heater plate 2 determines the subunit-to-subunit placement accuracy. While this approach has the advantages of being simple and inexpensive, it has some less than ideal characteristics. For example, errors in the delineation of the heater plate 2 can accumulate over the length of the pagewidth printhead (cumulative chip-misalignment). Another disadvantage is that there is no way to incorporate thermal expansion gaps between adjacent subunits to ease problems of thermal expansion mismatch between the material used to form the subunits and the substrate to which the array is bonded. Another problem is that physically butting the chips could damage the heater plate edges which have circuitry nearby. This problem also applies to image reading arrays which have photosites or other image-detecting components adjacent their edges. A further problem is that if the precision diced edge of the heater plate 2 is not perfectly vertical, chip to chip stand off can result (that is, the upper surfaces of the heater plates 2 which contain the resistive elements will not be contiguous with one another thus causing uneven spacing of nozzles along the length of the printhead array).
U.S. Pat. Nos. 4,690,391 and 4,712,018 to Stoffel et al, the disclosures of which are herein incorporated by reference, disclose a method and apparatus for fabricating long full width scanning arrays for reading or writing images. For this purpose, smaller scanning arrays are assembled in abutting end-to-end relationship, each of these smaller arrays being provided with a pair of V-shaped grooves formed by orientation dependent etching (ODE) and located in an upper, component containing face thereof. An aligning tool having predisposed pin-like projections insertable into the grooves on the smaller scanning arrays is used to mate a series of smaller arrays in end-to-end abutting relationship. Discretely located vacuum ports in the aligning tool are used to draw the smaller arrays into tight face-to-face contact with the tool until a suitable base is affixed to base surfaces of the aligned arrays and the aligning tool withdrawn.
A limitation of the method of Stoffel et al is that the formation of grooves on the circuit surface of each smaller scanning array subunit can render the fabrication of many subunits from a single wafer difficult. In particular, etching the alignment grooves after formation of the circuitry on the subunit can damage the circuitry, while formation of the grooves prior to the circuitry requires that a photoresist layer be deposited on the entire surface of the wafer which renders the wafer surface non-planar. It is difficult to accurately form circuitry on a non-planar wafer. Additionally, both of these processes require the steps of applying and patterning a photoresist layer which takes time and increases costs. Further, since the planar surfaces of the (100) silicon wafers used in the method of Stoffel et al are not perfect (100) planes (due to ingot sectioning tolerances of +/-1/2.degree. in the process which slices these wafers from a larger silicon ingot), the grooves created by etching are not perfectly shaped and therefore may not properly mate with the corresponding alignment structure located on the aligning tool. Although Stoffel et al state that mechanical machining can be used to form the grooves, it would be difficult and time consuming to form these grooves with a dicing saw in the circuit surface of each subunit since the grooves cannot extend across the entire surface of the subunit due to the circuitry located thereon.